Powder coating compositions utilized to produce varnish-like coatings on metals are applied by fluidized bed coating, flame-spraying, or electrostatic powder coating. The coatings thus produced have good flow properties, good adhesion, and high resistance to impact stress.
The use of amorphous and/or partially crystalline thermoplastic copolyester powders in fluidized bed coating, flame-spraying, or electrostatic powder coating methods is conventional. However, under practical conditions, such powder coating compositions have several important disadvantages in addition to their good properties.
Exemplary coating compositions of amorphous thermoplastic copolyesters are described in British Pat. No. 1,029,136 and DAS German Published Application No. 1,222,205 (U.S. Pat. No. 2,901,466). The British patent discloses copolyesters of terephthalic acid, ethylene glycol, and neopentyl glycol, in which the diol neopentyl glycol is 30-50 molar percent of the glycol units.
DAS No. 1,222,205 teaches that, inter alia, polyesters of terephthalic acid and 1,4-dihydroxymethylcyclohexane modified with succinic acid or neopentyl glycol, are suitable for coatings. Coating compositions produced therefrom yield transparent, varnish-like coatings having good adhesion and high luster.
A serious disadvantage of these coatings is low impact resistance. This deficiency becomes apparent, in cases of thick coatings and mechanical stress, by crack formation and inadequate Erichsen and impact depression values. Furthermore, amorphous powder coating compositions tend to form lumps when stored somewhat above room temperature. The shelf life of amorphous coating compositions thus is considered insufficient.
Partially crystalline, thermoplastic copolyesters based on polyethylene terephthalate or polybutylene terephthalate, in contrast, yield powder coating compositions having satisfactory shelf stability. In accordance with British Pat. No. 676,372, copolyesters of ethylene glycol and an acid component of about 70 molar percent of terephthalic acid and about 30 molar percent of an aliphatic dicarboxylic acid can be used in coating compositions. Powder coating compositions based on copolyesters of terephthalic acid, isophthalic acid, ethylene glycol, and 1,3-propanediol are described in DOS(German Unexamined Laid-Open Application) No. 1,905,825.
Powder coating compositions of copolyesters of terephthalic acid and ethylene glycol, modified with 5 molar percent of 2,2-di-[(4-hydroxyethoxy)-phenyl]-propane are disclosed in DOS No. 2,124,808. Coatings produced from these polyester powders have high hardness, but poor adhesion and impact depression values. Adequate improvement of these properties is not attained by quenching with cold water.
DOS No. 2,346,559 discloses powder coating compositions of partially crystalline copolyesters of 1,4-butanediol and terephthalic acid, isophthalic acid and/or an aliphatic dicarboxylic acid. Coatings from these polyesters exhibit good adhesion, improved flexibility, and good Erichsen depression. However, sufficient impact depression cannot be obtained with coatings from these copolyester powders, as will be demonstrated below in Comparative Examples A and B.
It is an object of this invention to provide an impact-resistant, highly adherent, lustrous powder coating composition based on saturated, high-molecular thermoplastic polyesters.